Low power cooling and flow inducement

ABSTRACT

A plasma actuator of the present disclosure comprises a plate electrode; a needle electrode positioned at a height above the plate electrode and a distance away from the plate electrode; and a voltage source connected to the needle electrode. Application of a signal from the voltage source to the needle electrode forms surface corona discharge around a tip of the needle electrode, and air flow driven by the corona discharge from the tip of the needle electrode is induced at a surface of the plate electrode. Accordingly, the air flow that is created can be used to cool a surface, such as a circuit board, a heat exchanger, etc.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to, co-pending U.S. Provisional Application entitled “LOW POWER COOLING AND FLOW INDUCEMENT,” filed on Jun. 2, 2015, and assigned application No. 62/169,620, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure is generally related to plasma actuators.

BACKGROUND

Atmospheric plasma driven active and passive flow control devices have been extensively studied in recent years. Applications of these devices range from the control of the laminar to turbulent transition to the drag reduction.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

FIG. 1(a) is a diagram of a needle actuation device setup in a wall jet configuration in accordance with an embodiment of the present disclosure.

FIG. 1(b) a diagram of a needle actuation device setup in a channel jet configuration in accordance with an embodiment of the present disclosure.

FIG. 1(c) is a diagram of a needle actuation device setup in a wall jet configuration with three needle electrodes in accordance with an exemplary cooling implementation of the present disclosure.

FIG. 1(d) is a diagram of a needle actuation device setup in a wall jet configuration with two needle electrodes in accordance with an exemplary vortex generating implementation of the present disclosure.

FIG. 2(a) is a diagram of an XZ-plane PIV velocity field for a needle actuation channel operated at positive 12 kV DC signal for an embodiment of the present disclosure.

FIG. 2(b) is a diagram of an XY-plane PIV velocity field for a needle actuation channel operated at positive 12 kV DC signal for an embodiment of the present disclosure.

FIG. 2(c) is a diagram XZ-plane velocity profile at an exit of a needle actuation channel for positive and negative 9 kV DC signal for an embodiment of the present disclosure.

FIG. 2(d) is a diagram XY-plane velocity profile at an exit of a needle actuation channel for positive and negative 9 kV DC signal for an embodiment of the present disclosure.

FIG. 3(a) is a diagram of electrical power consumption for various applied positive and negative voltages for an embodiment of the present disclosure.

FIG. 3(b) is a diagram of efficiency of a needle channel for various applied positive and negative voltages for an embodiment of the present disclosure.

FIG. 4 is a diagram illustrating the cooling effect of a needle actuation device setup in accordance with an embodiment of the present disclosure.

FIGS. 5(a) and 5(b) are diagrams showing vortical forces generated by vortex generation of needle actuation device setups in accordance with an embodiment of the present disclosure.

FIG. 6 is a flow chart diagram illustrating a method of plasma actuation in accordance with an embodiment of the present disclosure.

FIGS. 7(a)-7(b) are diagrams of an exemplary needle actuation device employed as part of a compact heat exchanger assembly in accordance with embodiments of the present disclosure.

FIG. 8 is a diagram of representative flow patterns for the compact heat exchanger assembly of FIGS. 7(a)-7(b).

FIGS. 9(a)-(b) are diagrams of cooling performance parameters showing improvements in dimensional (h) and non-dimensional (Nu) convective heat transfer coefficients for the compact heat exchanger assembly of FIGS. 7(a)-7(b).

DETAILED DESCRIPTION

Advantages of dielectric barrier discharge (DBD) driven plasma actuators include lack of moving parts, fast response, small scale, surface compliance, and ease of construction and application. Major disadvantage of these devices is that their energy conversion efficiencies are extremely low (˜0.1%). This is because of a large amount of energy loss in dielectric heating and in light emission. Also, the momentum injection to the neighboring flow is largely limited due to high viscous losses encountered in traditional DBD wall jets. As compared to DBD, direct current (DC) corona discharge only generates minimal glow near the surface of powered electrode without using any dielectric material. Thus, the energy loss mentioned above can be avoided. However, the energy conversion efficiencies of DC corona devices remain less than 1%. This is again due to excessive viscous loss from the thin wall shear layer or/and discharge loss near the wire surface. In contrast, a channel configuration with a needle electrode can be employed to minimize the viscous loss from the plasma injected momentum, in accordance with certain embodiments of the present disclosure.

In accordance with embodiments of the present disclosure, atmospheric DC corona discharge devices include a powered electrode and a grounding electrode separated by air. The powered electrode has a sharp tip or round edge with a small radius of curvature, in some embodiments, where the grounding electrode often has a large smooth surface. Application of high-voltage DC signal (HVDC) to the powered electrode forms surface corona discharge around the sharp tip or the round edge where the maximum electric field is generated. The grounding electrode is placed some distance away to generate desired electric field to induce the ions formed by impact ionization. Then, collisional momentum transfer between ions and neutral particles occurs in the space between the electrodes. Both positive and negative DC voltages can be used to induce flow with slightly different mechanisms. For the positive corona, ionization region is generated by anode electron avalanche near the electrode. Then, positive ions are repelled toward the cathode by Coulomb force. In the case of negative corona, negative ions are created by electron attachment and then repelled toward the anode.

Accordingly, the present disclosure explores a class of DC corona needle actuation devices that has been developed to generate moderate directed airflow with extremely low power consumption, and thus improve the energy conversion efficiency by an order of magnitude than its DBD counterpart. Particular electrode arrangements for these devices show great benefits for cooling and vortex generation applications.

Motivated by the advantages of DC corona discharge, steel needles and copper plates are used to generate jet flow and vortex in one embodiment. For a wall jet configuration 110 shown in FIG. 1(a), a high voltage powered steel needle electrode 120 is placed in the upstream side of a plate electrode 130. Such an arrangement allows air flow driven by the corona discharge from the tip of the needle electrode to the plate with either positive or negative DC signals. The configuration shown in FIG. 1(a) shows a single needle electrode; however, multiple needles evenly separated in Y-direction (span-wise) are employed in cooling and vortex generation embodiments shown in FIG. 1(c) and FIG. 1(d) respectively. Accordingly, the relative positioning or orientation of the needle electrode with respect to the plate electrode may change for different embodiments in accordance with the present disclosure. For example, the respective orientation may be parallel in one embodiment, may be perpendicular in another embodiment, may be at a 30 degree angle in one embodiment, etc.

For a channel jet configuration 150 shown in FIG. 1(b), a high voltage powered steel needle 160 is placed in between two plate electrodes 170, 180 to form two wall jets. With this configuration, direct momentum transfer into bulk flow may generate less wall shear than the wall jet configuration. Since a needle to plate configuration will generate a 3D flow structure, multiple needles can be employed to generate a vortex, in some embodiments. Also, in some embodiments, non-straight needle electrodes may be featured. For example, a needle electrode having one or more of turns or a serpentine shape is utilized in one embodiment.

A description of experiment setups utilized in FIG. 1(a) and FIG. 1(b) is described below. Powered needle electrodes 120, 160 and plate electrodes 130, 170, 180 are separated by gap g in an X-direction and height H in a Z-direction. And, the width of the plate electrode 130, 170, 180 (in an X-direction) is denoted as w in the configuration. In the test cases, Y-direction (span-wise) length of plate electrodes is set to be 10 cm. For the channel jet configuration shown in FIG. 1(b), only one needle electrode 160 is mounted. However, additional needle electrodes may be featured in other embodiments.

In the respective configurations, the needle electrode(s) are powered with high-voltage DC supply (TREK Model 30/20A High-Voltage Power Amplifier) and the plate electrodes are, but not limited to being, grounded. For example, in one embodiment, a resistor (e.g., a variable resistor or a fixed resistor) is connected between a plate electrode and the ground. In another embodiment, the plate electrode is floating as opposed to being connected to ground.

As shown, the DC signal (HVDC) used is continuous with constant amplitude. For testing purposes, the needle electrodes have a radius of curvature ρ_(e)=100 μm, and the plate electrodes have thickness of 60 μm and width w=5 mm.

During testing, electrical parameters were measured using an oscilloscope (Tektronix DPO2014) and a digital multi-meter (BK Precision 5491A). DC Voltage was measured using a passive probe (Tektronix Model P6015A) with an attenuation of 1000× through the oscilloscope. The oscilloscope has a maximum sampling rate of 1 GSa/s at a bandwidth of 100 MHz. The range of voltages used in the power and efficiency tests (described below) is from 6 kV to 12 kV. DC current was measured by the digital multi-meter connected in series in a circuit of the jet configuration being tested. The digital multi-meter can measure the DC current with resolution of 1 μA. The corona discharge used during testing was in glow mode. Accordingly, the voltage and current of the circuit was approximately constant. Average voltage and average current were obtained from the measurement to calculate average electrical power using V-I method, P_(elec)=VI, where V and Ī are average voltage and current measured through a 10 second period, respectively.

For testing purposes, a LaVision Particle Image Velocimetry (PIV) system was used to collect the velocity profile of the needle actuation device 110, 150. A laser sheet used to illuminate Ondina oil seeding particles was created by a 532 nm Nd:YAG (New Wave Research Model Solo PIV II 30) laser generator fitted with a divergent cylindrical lens. Images of illuminated seeding particles were captured by a Phantom 7.3 high speed camera which has a resolution of 800×600 pixels. LaVision's Davis 7.2 software was used to control both laser generator and high speed camera by generating external trigger through Model PTU-9, programmable timing unit (synchronization resolution of 10 ns with <1 ns jitter). In this series of tests, the PIV system was employed to capture XY-plane and XZ-plane velocity profile for 3-D velocity visualization of the needle actuation channel and YZ-plane velocity profile to show vortex generation. According to Durscher's study, statistical convergence for the plasma jet is within 300 samples (image pairs for correlation). Nonetheless, 1000 samples were taken or collected to eliminate the deviation.

According to the velocity profile generated from PIV system, electro-mechanical energy conversion efficiency was obtained to evaluate the performance of the needle actuation device. In mechanical engineering, the induced mechanical power of the airflow inside a duct is given by

${P_{mec} = {\frac{\rho}{2}{\int{\int{u_{x}{A}}}}}},$

where ρ is the air density, u_(x) is the velocity at X-direction, and the 2-D integration covers all the area A with nonzero velocity. Electro-mechanical energy conversion efficiency can be calculated as the ratio of the induced mechanical power of the airflow to the electrical power consumed in the applicable circuit configuration 110, 150 by the equation η=P_(mec)/_(elec).

Multiple configurations were used to experimentally investigate the performance and applications of a low power needle actuation device 110, 150. Needle actuation channel shown in FIG. 1(b) was used to characterize its capability of airflow inducing. In the channel setup (FIG. 1(b)), the following parameters were used: gap g=15 mm and channel height 2H=5 mm. The PIV was employed to study the 3-D flow structure of the channel and electro-mechanical energy conversion efficiency was calculated to evaluate the channel performance.

For the testing of film cooling effect and vortex generation, the wall jet configuration was utilized with multiple needle electrodes (see FIG. 1(c) and FIG. 1(d)). In film cooling tests, three needle electrodes 121, 122, 123 separated by 2 cm (in the Y-axis) were placed at height H=2.5 mm (in the Z-axis) with gap g=15 mm (in the X-axis) as represented in FIG. 1(c). During testing, at room temperature 21° C., one rubber heater (24V, 12 W with surface area 5×5 cm²) placed adjacent to the plate electrode 130 was used as heat source simulating an integrated circuit (IC) chip. Four thermocouples were positioned on the corners of a 3×3 cm² square beginning from 2 cm downstream of the plate electrode 130. Time varying temperature data was obtained from the average of all four thermocouples, which represents the surface temperature of a 3×3 cm² area.

In vortex generation tests, two needle electrodes 125, 126 separated by 3 cm (in the Y-axis) were placed at the same height H=2.5 mm (in the Z-axis) with a gap g=15 mm (in the X-axis). In order to control the direction of the vortex, the applied DC signals for these two needles were made to be, but not limited to being, slightly different for these tests. Alternatively, in some embodiments, the same DC signal is applied to the plurality of needle electrodes.

FIGS. 2(a) and 2(b) shows the PIV data for the velocity field induced by the needle actuation channel powered with positive 12 kV DC signal as obtained during testing. Both XZ-plane (FIG. 2(a)) and XY-plane (FIG. 2(b)) velocity fields were obtained to demonstrate the 3-D airflow structure generated by a single needle actuation channel configuration (see FIG. 1(b)). The figures show the maximum velocity of 2.6 m/s which was the highest positive voltage applied to the channel before transition from steady glow mode to unsteady streamer mode. FIG. 2(a) shows strong jet flow cover the entire channel exit in the Z-direction with distinct entrainment effects on both sides of the channel. In XY-plane velocity field of FIG. 2(b), induced airflow covers about 2 cm wide region at the channel exit in the Y-direction. Note that only one needle electrode was powered to create this velocity field. This character can be used in generating uniform flow using needle arrays, such as a linear array of needle electrodes. In addition, strong airflow penetration effect into quiescent surroundings can be noticed in both XY- and XZ-plane. These qualities suggest that this type of DC needle channel jet configuration (FIG. 1(b)) has great potential in flow control and propulsion applications.

Next, FIGS. 2(c) and 2(d) display the velocity profile at the exit (X=0 mm) of the single needle actuation channel (see FIG. 1(b)) under different operating signals. As shown in FIG. 2(c), velocity profiles generated by a negative DC signal have very small boundary layers near the plate electrode 170, 180 and a nearly flat middle region. This indicates that the momentum transfer from negative ions to neutral particles mainly happens near the wall. Thus, this is strong evidence that electron attachment may mainly happen outside the ionization zone near the tip of the needle electrode 160.

On the other hand, velocity profile of a positive signal shows a sharp peak at the middle of the channel as displayed in FIG. 2(c), which suggests the momentum transfer between positive ions and neutral particles mainly happens near the ionization zone around the tip of the needle electrode 160. For the span wise (Y-axis) velocity distribution shown in FIG. 2(d), both positive and negative signals generate similar effective region width at 9 kV. Additionally, induced mechanical power was calculated according to the 2-D integration on both Y-axis and Z-axis velocity profiles. In this integration, PIV velocity profiles at Y-axis and Z-axis are expanded to YZ-plane using the proportional method.

Referring now to FIG. 3(a), the channel electrical power consumptions and maximum velocities are shown as a function of applied voltage for both positive and negative signals in log scale. The velocity profile generated by a negative DC signal increases faster as the absolute signal amplitude increases than that by a positive DC signal. A similar trend can also be observed in power consumption curves. Note that the power consumed by the needle actuation channel (see FIG. 1(b)) is only several mW, which is extremely low compare to other plasma driven flow devices. Power fits based on linear regression are applied on either of the cases and the results show that the power of positive signal is 5.6 as well as the power of negative signal is 9.4. Both models fit very well with the data with coefficient of determination, R²>0.99. Similar to the Z-axis velocity profile, it is clear that the power consumption increases faster as the absolute signal amplitude increase when the signal is negative. This difference is mainly due to the different mechanism of generating working ions. For the positive signal, positive ions are mainly generated inside the ionization zone near the needle tip. On the other hand, negative ions are mainly generated by electron attachment outside the ionization zone. When the absolute signal amplitude increases, positive ion generation is limited by the size of the ionization zone while many more electrons can be used to generate negative ions outside the ionization zone.

Electro-mechanical energy conversion efficiency for both the positive and negative signals are displayed in FIG. 3(b). As shown, the maximum efficiency reached of the needle actuation channel configuration (see FIG. 1(b)) is 2.8% at positive 7.5 kV. This efficiency is one order of magnitude larger than the efficiency of a surface corona actuator and is about twice the efficiency of a wire to cylinder configuration. In both cases, the efficiency increases rapidly from 6 kV to 7.5 kV and then gradually decreases from 7.5 kV to 12 kV. This suggests that the higher the voltage magnitude, the more energy is wasted. One can also notice that the efficiency of the positive signal is about twice the efficiency of the negative signal at the same magnitude which may be explained by the wall shear due to the different flow structure mentioned above.

Not like other plasma driven flow methods, a needle actuation device 110, 150 can generate sufficient amount of flow with very low power consumption. This suggests that much less power is wasted in heat generation. So, this characteristic makes it a perfect film cooling device. To demonstrate, a wall jet configuration with three needle electrodes (see FIG. 1(c)) was used in the cooling test, and the time varying temperature curve for cooling effects is shown in FIG. 4.

The curve clearly shows the strong cooling effects of the needle actuation device 112 operating at different voltages. With increasing applied positive voltage, the temperature at the surface reduces faster and faster. At positive 12 kV, three electrode needles 121, 122, 123 are observed to cool a 3×3 cm² area from 62° C. to 47° C. in 80 seconds. Additionally, the total power for each voltage applied shown in the legend of FIG. 4 remains at mW level which gives great potential for cooling applications without the need to ground the surface to be cooled and/or utilize a chamber to contain the cooling airflow. Accordingly, a flow can be created to cool the surface, such as a circuit board, a heat exchanger, etc., which is holding the plate electrode 160 and/or other devices disposed on the surface, such as integrated circuits and chips, among others.

In one embodiment, a needle actuation device 114 (FIG. 1(d)) is also used to generate vortex near the surface. By slightly changing the gap g between needle and plate electrodes, a vortex is generated between two needle electrodes 125, 126 to which the direction of the vortex can be controlled. As displayed in FIG. 5(a), needle electrode #1 (125) at Y=−15 mm was powered with 10 kV DC signal while an 8 kV DC signal was applied on needle electrode #2 (126) at Y=15 mm during a first series of tests. After powering both needle electrodes during testing, an anti-clockwise vortex was captured using PIV system at X=10 mm. Note that the airflow generated by needle electrode #1 (125) is stronger near the wall so that it pushes the airflow generated by needle electrode #2 (126) and then forms an anti-clockwise vortex near the wall. Accordingly, respective electrodes may be positioned in selective locations and/or powered at varying voltages to create variations in the flow structure and create three dimensional vortices.

For a second series of tests, needle electrode #2 (126) at Y=15 mm was powered with 10 kV DC signal which is higher than the 8 kV DC signal applied on needle electrode #1 (125) at Y=−15 mm, as indicated by FIG. 5(b). Since stronger airflow near the surface is generated on the needle electrode #2 side, a vortex is generated that rotates in a clockwise direction. Accordingly with the low power consumption character of this device configuration 114, the power used to generate this vortex is only 12 mW in total. Thus, this type of needle actuation device has great potential to drag reduction, flow mixing, and many other flow control applications.

In summary, the power consumptions for both the wall-jet and channel jet configurations of the DC needle actuation device are only at mW level. Combined with other advantages of plasma driven flow control methods, this type of low power needle actuation device has great potential for flow control, cooling, drag reduction, etc. As shown above, needle actuation devices of the present disclosure can essentially be driven at very low power (e.g., orders of magnitude less than standard plasma actuators) and still generate sufficient flow to generate a desired velocity in a channel, to cool a desired surface, and/or to create three-dimensional vortical structures.

For an embodiment of the channel jet needle actuation device (see FIG. 1(b)), momentum was directly injected by the needle-to-plate electrode configuration into bulk airflow of the channel with minimal loss due to wall shear stress. As shown from testing, both positive and negative high voltage DC signal can be used to generate a strong ionic flow in the same direction. However, positive ionic flow shows a sharp peak in the center while negative ionic flow has higher velocity near the plate electrode 130. This difference indicates that the momentum transfer mainly happens near the tip of needle electrode 160 under a positive signal and near the plate electrode 130 under a negative signal. As shown in testing, strong plasma jet from a single needle actuator (see FIG. 1(b)) can cover a 2 cm range in span-wise direction (Y axis) with a high velocity core. This makes it possible to generate a long flow jet using a needle array. The electro-mechanical energy conversion efficiency of such a device can reach up to 2.8%, almost 30 times the standard DBD channel or 4 times the wire DBD channel. Strong entrainment effect was also observed during testing.

The cooling effect and vortex generation were also investigated using multi-needle wall jet configurations (see FIG. 1(c) and FIG. 1(d) respectively). The testing on cooling effects shows that a needle actuation device (FIG. 1(c)) cooled down a hot area up to 15° C. using mW energy in 80 seconds. Additionally, a multi-needle wall jet configuration (FIG. 1(d)) was also used to generate a controllable vortex between two needle electrodes 125,126 with different voltages. Therefore, from the various tests performed, this type of low power needle actuation device shows great potential to be developed for many prospective applications.

FIG. 6 illustrates a method of plasma actuation in accordance with an embodiment of the present disclosure. An exemplary method comprises providing (610) a voltage source; providing (620) a needle electrode connected to the voltage source; providing (630) a plate electrode, wherein the needle electrode is disposed at a height above the plate electrode and at a distance away from the plate electrode; applying (640) a signal from the voltage source to the needle electrode thereby forming a corona discharge around a tip of the needle electrode; and inducing (650) air flow at a surface of the plate electrode driven by the corona discharge from the tip of the needle electrode. For various embodiments, the air flow is abundant to generate a desired velocity in a channel, to cool a desired surface, and/or to create three-dimensional vortical structures, among other possible applications.

One such application, among others, is with compact heat exchangers (CHE) that employ surface geometries that have high heat transfer properties, such as plate-fin and tube-fin exchangers, among others. For example, shell and tube heat exchangers are popular in oil refineries and chemical process industries, and fin heat exchanger are often used in the aerospace industry and in cryogenics.

In general, the weight of a compact heat exchanger depends on the size (length) of the fins/tubes, which strongly depends on the convective heat transfer coefficient h of the neighboring fluid—The higher the h, the smaller the length of the tube/fin needed. One current disadvantage of most of the compact heat exchangers (CHE) to date is their weight and/or size, which is a serious issue for many applications especially if they demand portability. Improvements in the overall power management approach can be made so as to reduce the weight associated with man-portable energy storage.

For example, in a thermoelectric heat exchanger, weight is driven primarily by two factors, for a given power output: the high/low temperature ratio, and the size and weight of the heat exchange subsystems. These two effects are coupled, especially for the heat rejection region, as optimized cold-side heat exchangers strike a balance between fin surface area requirements and the temperature difference between the CHE cold plane and ambient.

There are several ways one can improve heat transfer coefficient h. For example, changing the fluid from air to water (increasing the conductivity of the fluid) will improve the h by several orders of magnitude. However, that will also significantly increase the average density and cooling arrangement complexity of the CHE system. In a packed fin arrangement, nearly 90% of the fluid passage may be restricted causing slowing down (if not stagnation) of the flow just behind individual fin structures, resulting in a huge resistance to the flow. This requires additional pumping (blower) power to improve the convective cooling performance. Even then, the trapped fluid in the wake region may not be sufficiently forced to effectively convect heat out from that region.

Accordingly, a needle electrode (e.g., a microscale needle) in a needle actuation device setup of an embodiment of the present disclosure is introduced in the wake region to apply an electrically induced body force to motivate the stagnant fluid. For example, in one embodiment, the needle actuation device comprises a needle electrode in a form of a thin wire running parallel or substantially parallel to a fin structure. The electrical arrangement may use negligible fraction (milliwatts) of the CHE power budget. This will allow us to reduce the weight of the CHE drastically. It is anticipated that increasing h in such a heat exchanger design will result in effectively lighter, shorter fins or heat pipes that will significantly reduce its weight.

Embodiments of such a needle actuation device in accordance with the present disclosure can enable a lighter, more compact heat exchanger with a smaller temperature difference. Accordingly, an embodiment of the present disclosure utilizes micro-scale flow control technology within a heat exchanger to augment the heat transfer coefficient, such as in the wake region behind fin(s). With the cold plane closer to ambient temperature, the CHE efficiency is improved, reducing the required heat rejection rate, which further reduces the size and weight of the cold-side (and hot-side) heat exchanger.

FIGS. 7(a)-7(b) illustrate an assembly of a compact heat exchanger utilizing a pin fin array with needle action devices. In the figures, a pin fin array 710 of a CHE is and is attached to a fin base plate 715 (cold plane) of a CHE, with an adjacent part of an outer shroud composed of an electrical insulator (dielectric) 720, in accordance with one embodiment. An array of fine electrically-conducting needle actuation devices (also referred as an electric cilia) 730 is supported from the dielectric wall 720 such that the electric cilia 730 are positioned near the wake region of each fin 710 to provide an electrically induced body force or flow 740. In so doing, a moderate voltage difference can be applied to the electric cilia 730 thereby generating a high electric field due to the close spacing of the positive and negative electrodes (anode and cathode). The electric field generates a plasma, and interaction with its charge carriers induces a significant body force in the fluid, in accordance with the present disclosure.

This concept for promoting effective cooling enhancement of a CHE with an array of electrically biased cilia 730 can be employed behind circular or rectangular fins or heat pipes, in various embodiments. As a test, a numerical simulation using air was performed to explore the concept of using electric cilia 730 just downstream of an array of 1 mm diameter cylindrical fins 710. A significant modification of the fluid separation point behind the fin 710 is easily attainable, as shown by the representative result of FIG. 8.

In FIG. 8, a composite pressure and streamline map shows the flow field with the needle actuation devices or actuator 730 turned on (Right Side of FIG. 8) vs. with the actuator 730 turned off (Left Side of FIG. 8). Clearly, the size of the separation bubble decreases about 30% with the actuator 730 engaged, and the attached flow extends farther along the downstream side of the fin 710. These results are isothermal, but the modified fluid pattern is estimated to increase the circumferentially-averaged film coefficient by at least 20%. An estimation of the influence of electric cilia 730 for added convective cooling (h and Nu (Nusselt Number)) is shown in FIGS. 9(a)-9(b). The results show a significant increase of both these convective parameters just downstream of the cylindrical fins 710 by over 25%. Based on Equation (2) below, this indicates an effective reduction of the size of the fin 710 (heat pipe) by approximately 12.5%. Also, if electric cilia induced film cooling is used to effectively convect the trapped heat in and around a heat pipe and fluid is designed to flow around the fin structures 710, the added fluid volume can replace a significant fraction of the solid conductive wall of the fin/pipe, reducing a weight of the CHE.

Consider the characteristic length l (m) equation of the fin,

$\begin{matrix} {{ = \sqrt{\frac{{kA}_{c}}{hP}}},} & (1) \end{matrix}$

where, k is the conductivity (W/mK), h is the convective heat transfer coefficient (W/m²K), A_(c) is the cross-sectional area (m²) and P is the perimeter of the fin (m). From Equation (1), it is easy to see that for same conductivity and fin cross-section any increase in h reduces the length of the fin l, i.e.,

$\begin{matrix} {{\frac{_{2}}{_{1}} = \sqrt{\frac{h_{1}}{h_{2}}}},} & (2) \end{matrix}$

which directly helps reducing the weight of the cooling system. Also, the net cooling heat transfer q(W) is:

$\begin{matrix} {{q = \frac{\Delta \; T}{R}},} & (3) \end{matrix}$

where ΔT is the change in temperature (K) and R is the total (conductive, convective and radiative) thermal resistance of the system. The convective resistance is inversely proportional to h. Thus, increasing the film coefficient h has a drastic impact on the cooling heat transfer process and overall weight (average density) of the CHE.

Note that the disclosed flow control solution has been experimentally demonstrated for enhancing convective heat transfer coefficient on a flat plate by up to 57.7% using milliwatt level power. Embodiments of a needle actuation device in accordance with the present disclosure can be readily combined with future advances in fin material conductivity and heat pipes to yield even greater benefit. The disclosed techniques are easy to modulate, so a time-varying body force via the plasma can be imposed in certain embodiments of the needle actuation devices. This may couple to vortex formation, enhancing film coefficient throughout the flow field, not just in the wake region.

In certain embodiments, the power used to operate the needle actuation device should be a negligible fraction of the CHE output. Thus, in some embodiments, the power supply can be integrated with the CHE controls, resulting in a highly compact, robust, body-conformable package. Further, the plasma control approach is unaffected by ambient temperature, orientation, or heat source proximity. Since there are no moving parts, a rugged device can be designed for use in many types of environments, including the loads and shocks imposed by a soldier in the field, as an example.

It should be emphasized that the above-described embodiments of the present disclosure are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the present disclosure. Many variations and modifications may be made to the above-described embodiment(s) without departing substantially from the spirit and principles of the present disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims. 

Therefore, at least the following is claimed:
 1. A plasma actuator comprising: a plate electrode; a needle electrode positioned at a height above the plate electrode and a distance away from the plate electrode; and a voltage source connected to the needle electrode, wherein application of a signal from the voltage source to the needle electrode forms surface corona discharge around a tip of the needle electrode, wherein air flow driven by the corona discharge from the tip of the needle electrode is induced at a surface of the plate electrode.
 2. The plasma actuator of claim 1, wherein the plate electrode is connected to ground.
 3. The plasma actuator of claim 1, wherein the voltage source applies a positive DC signal to the needle electrode.
 4. The plasma actuator of claim 1, wherein the voltage source applies a negative DC signal to the needle electrode.
 5. The plasma actuator of claim 1, wherein the needle electrode comprises steel material.
 6. The plasma actuator of claim 1, wherein the plate electrode comprises copper material.
 7. The plasma actuator of claim 1, further comprising an additional plate electrode positioned a height above the needle electrode and a distance away from the needle electrode, wherein two wall jets are formed from the corona discharges of the needle electrode.
 8. The plasma actuator of claim 1, further comprising at least one additional needle electrode positioned as a linear array with the needle electrode at the height above the plate electrode.
 9. The plasma actuator of claim 8, wherein the needle electrode is powered at a voltage signal that is different from a voltage signal that powers the additional needle electrode.
 10. The plasma actuator of claim 1, wherein the distance is 15 mm and the height is 2.5 mm.
 11. The plasma actuator of claim 1, wherein a width of the plate electrode is 10 mm.
 12. A method of plasma actuation comprising: providing a voltage source; providing a needle electrode connected to the voltage source; providing a plate electrode, wherein the needle electrode is disposed at a height above the plate electrode and at a distance away from the plate electrode; applying a signal from the voltage source to the needle electrode thereby forming a corona discharge around a tip of the needle electrode; and inducing air flow at a surface of the plate electrode driven by the corona discharge from the tip of the needle electrode.
 13. The method of claim 12, further comprising connecting the plate electrode to ground.
 14. The method of claim 12, wherein the signal applied to the needle electrode is a positive DC signal.
 15. The method of claim 12, wherein the signal applied to the needle electrode is a negative DC signal.
 16. The method of claim 12, further comprising providing an additional plate electrode positioned a height above the needle electrode and a distance away from the needle electrode, wherein two wall jets are formed from the corona discharges of the needle electrode.
 17. The method of claim 12, further comprising providing at least one additional needle electrode positioned as a linear array with the needle electrode at the height above the plate electrode.
 18. The method of claim 17, wherein the needle electrode is powered at a voltage signal that is different from a voltage signal that powers the additional needle electrode.
 19. The method of claim 17, wherein the air flow induced by the corona discharge of the needle electrode and the additional needle electrode comprises one or more vortices.
 20. The method of claim 19, further comprising controlling direction of rotation of the one or more vortices by applying different voltage signals to the needle electrode and the additional needle electrode.
 21. The method of claim 12, further comprising positioning the plate electrode adjacent to a circuit component so that the circuit component is cooled by the air flow induced by the corona discharge.
 22. The method of claim 21, wherein with an increase in applied positive voltage at the signal, a temperature at a surface adjacent to the plate electrode reduces.
 23. The method of claim 12, further comprising positioning the plate electrode adjacent to a fin extending from a base of a heat exchanger, wherein the fin dissipates heat from a surface of the base to ambient fluid surrounding the at least one fin; and wherein the air flow driven by the corona discharge from the tip of the needle electrode motivates movement of the ambient fluid surrounding the fin.
 24. A heat exchanger system comprising: at least one fin extending from a base plate of a heat exchanger, wherein the at least one fin dissipates heat from a surface of the base plate to ambient fluid surrounding the at least one fin; and at least one needle actuation device positioned near a wake region of the at least one fin that is adapted to provide an electrically induced body force to motivate movement of the ambient fluid surrounding the at least one fin.
 25. The system of claim 24, wherein the at least one fin is rectangular in shape.
 26. The system of claim 24, wherein the at least one fin is circular in shape.
 27. The system of claim 24, wherein the at least one needle actuation device comprises: a needle electrode positioned at a height above a plate electrode and a distance away from the plate electrode; and a voltage source connected to the needle electrode, wherein application of a signal from the voltage source to the needle electrode forms surface corona discharge around a tip of the needle electrode.
 28. The system of claim 24, wherein the at least one needle actuation device comprises a needle electrode in a form of a thin wire running substantially parallel to the at least one fin. 